The present invention relates generally to a humidity control application for a cooling system. More specifically, the present invention relates to a method for performing humidity control using a hot gas reheat coil in a two stage cooling unit.
Some refrigeration systems use a hot gas re-heat control to perform humidity control for an interior space. A hot gas re-heat coil is placed immediately adjacent to an evaporator and receives air from the evaporator. When humidity control is required, the refrigerant from the compressor is passed through the re-heat coil instead of the condenser so that the re-heat coil can operate as a condenser to heat and further dehumidify the air passing over the re-heat coil. Once the system enters a humidity control mode, there is essentially no cooling of the air for the interior space because the air being cooled by the evaporator is then being heated by being passed over the re-heat coil. Some examples of previous systems for providing humidity control are provided below.
U.S. Pat. No. 6,553,778, hereafter the '778 Patent, describes a multi-stage cooling system having a plurality of independent refrigeration circuits to provide a plurality of cooling capacities. The refrigeration circuits have different capacity compressors, typically a larger capacity compressor and a smaller capacity compressor, which can be controlled and cycled by a controller to obtain different cooling capacities. The controller is also used for humidity control. The control system operates in a temperature control mode and enters a dehumidification mode only if the temperature control mode is unsuccessful at maintaining a desired humidity level. When dehumidification is required, the control system first attempts to control humidity by engaging the larger capacity compressor. If that is unsuccessful, then the larger capacity compressor is operated continuously with re-heaters to maintain a desired temperature and, if necessary, the lower capacity compressor can be cycled for temperature control. For a smaller load requirement in the system, the larger capacity compressor is cycled on in response to a call for cooling and/or dehumidification, a re-heater is cycled on in response to a call for dehumidification without a call for cooling, and a hot gas by-pass is engaged when there is a call for cooling without a call for dehumidification. One disadvantage of the '778 Patent is that a reheater (or heater circuit) separate from the refrigerant circuit is used to provide dehumidification.
U.S. Pat. No. 4,813,474, hereafter the '474 Patent, describes an air conditioner that provides dehumidification. The air conditioner includes a refrigerant circuit or cycle with a variable capacity compressor and a reheater arranged in association with the indoor heat exchanger. The variable capacity compressor and reheater are controlled based on a temperature differential to provide cooling and dehumidification. For a large temperature differential, e.g. >3° C., only the variable capacity compressor is operated under high capacity to provide cooling. As the temperature differential becomes smaller, both the compressor and reheater are operated at varying levels to provide the appropriate amounts of re-heat for a given temperature differential. One disadvantage of the '474 Patent is that a reheater (or heater circuit) separate from the refrigerant circuit is used to provide dehumidification.
U.S. Pat. No. 5,752,389, hereafter the '389 Patent, describes a cooling and dehumidification system that uses refrigeration re-heat for temperature control. The system has a standard refrigeration circuit with a re-heat coil connected in parallel with the outdoor coil and positioned adjacent to the indoor coil. A portion of the refrigerant is diverted from the outdoor coil to the re-heat coil to re-heat the air during the dehumidification mode, while the remaining refrigerant flows according to the regular refrigerant circuit. The amount of re-heat provided by the re-heat coil is determined in response to a sensor measurement in the discharge air and a set-point value. One disadvantage of the '389 Patent is that the amount of available humidity control is based on the discharge air temperature.
U.S. Pat. No. 5,345,776, hereafter the '776 Patent, describes a heat pump system that has two indoor heat exchangers connected by an expansion valve in a single refrigeration circuit. During heating and cooling modes, both indoor heat exchangers function as condensers and evaporators, respectively. During dehumidification mode operation, the first indoor heat exchanger cools and dehumidifies the air and the second indoor heat exchanger heats the cooled air before it is supplied to the room. One disadvantage of the '776 Patent is that humidity control cannot be provided during a cooling operation.
U.S. Pat. No. 5,129,234, hereafter the '234 Patent, describes a humidity control for regulating compressor speed. The humidity control is used with a heat pump system having a two-speed compressor. The humidity control is a slave to the temperature control of the heat pump system in that the humidity control is non-functional when the temperature demand has been satisfied. The humidity control can override the temperature control to provide enhanced dehumidification. The humidity control will typically override a command for low speed compressor operation with a high speed command when certain predetermined humidity criteria are not satisfied. One disadvantage of the '234 Patent is that humidity control cannot be provided without providing cooling to an interior space.
Therefore, what is needed is a system and method that can provide both humidity control and some cooling to the interior space in response to demands for both humidity control and cooling.